The present invention relates to an improvement in a magneto-optical recording medium or, more particularly, to an improvement for greatly increasing the recording sensitivity of a magneto-optical recording medium.
Along with the rapid progress of the information-predominant society, electronic information recording media for storage of information are required to have more and more increased recording density and larger and larger capacity for information storage. In this regard, magneto-optical recording media are highlighted in respect of the rewritability of information and replaceability of one medium with another so that extensive and intensive investigations are now under way to improve the performance of magneto-optical recording media in several aspects.
Magneto-optical recording media in general have a laminar structure with a transparent substrate plate and layers successively formed thereon including a magnetic recording layer and a metallic reflecting layer with, optionally but in most cases, layers of an inorganic dielectric material intervening, one, between the substrate plate and the magnetic recording layer and, the other, between the magnetic recording layer and the metallic reflecting layer.
The magnetic recording layer in the magneto-optical recording media under current use is mostly formed from an alloy of transition metals including at least one kind of the rare earth elements such as an alloy of terbium, iron and cobalt. A problem in such a recording layer of a rare earth-based alloy is that, since the Kerr rotation angle of such a recording layer is relatively small, the C/N ratio in the reading-out of the recorded signals cannot be high enough because the C/N ratio is proportional to the product of the Kerr rotation angle and the square root of the reflectivity. It is known that the Kerr rotation angle can be increased by providing an interference layer or a reflecting layer so that a typical magneto-optical recording medium has a laminar structure consisting of a transparent substrate plate, a first dielectric layer, a magnetic recording layer, a second dielectric layer and a metallic reflecting layer as is mentioned above.
Needless to say, the properties of the reflecting layer has a great influence on the performance of the magneto-optical recording medium. In a magneto-optical recording medium having a structure in which the second dielectric layer in the above mentioned typical one is omitted, for example, the angle of optical rotation can be increased by utilizing the Faraday rotation occurring when the light transmitting through the recording layer and reflected on the reflecting layer again transmits the recording layer. In a typical magneto-optical recording medium having the second dielectric layer, the angle of optical rotation in read-out can be further increased by the multiple reflection on the second dielectric layer so as to increase the C/N ratio in read-out. A requirement for the reflecting layer is that the reflectivity thereof should be as high as possible as is self-explanatory. Besides, the reflecting layer is required to have a thermal conductivity as small as possible. The reason therefor is that, since recording by the incident light to the recording layer takes place when the temperature of the layer is increased approaching the Curie temperature or the compensation temperature by the energy of light absorbed by the layer, a larger thermal conductivity results in retardation of the temperature increase of the layer by the accumulation of heat as a consequence of an increase in the thermal diffusion.
A typical and most conventional metallic material of the reflecting layer is aluminum which exhibits, when it is pure aluminum, a reflectivity of as high as about 86% at a wavelength of around 800 nm which is the wavelength of the laser beams emitted from a semiconductor laser widely used in magneto-optical disc drives. The advantage of a reflecting layer of aluminum with such a high reflectivity is partly cancelled by the large thermal conductivity of aluminum which is as large as 150 W/m.multidot.K or larger resulting in an insufficient recording sensitivity, which is still lower when the velocity of the disc drive is increased and consequently the linear velocity of the light beam is increased resulting in a decrease of the applied energy to the recording layer per unit area per unit time.
A proposal is made in Japanese Patent Kokai 1-264649 according to which the reflecting layer of a magneto-optical recording medium is formed from palladium or platinum which is a metal having a small thermal conductivity of each about 70 W/m.multidot.K. These metals, however, are defective in respect of the low reflectivity of light which is about 60 to 70% so that the C/N ratio in reading-out of a magneto-optical recording medium cannot be large enough.